feat(uploader): add ryzencdn local-disk uploader (POST multipart + GET serve)

Shirokami-API source uploads to Ryzumi S3 (s3.ryzumi.vip) which is DNS-dead
from this VPS. Implemented a local-disk uploader keeping the source's response
shape {success, url, fileName, size}:
- POST /uploader/ryzencdn (multipart 'file' field, magic-byte ext detection)
- GET /uploader/file/{name} (path-traversal safe, MIME by extension)
Files stored under /var/lib/scraper/uploads (owned by scraper user).

src/application/uploader.rs

@@ -0,0 +1,13 @@
+//! Application use-cases for the uploader.
+
+use crate::domain::error::ScrapingError;
+use crate::infrastructure::repository::uploader as Repo;
+use serde_json::Value;
+
+pub fn upload(buffer: &[u8], file_name: &str) -> Result<Value, ScrapingError> {
+    Repo::save_file(buffer, file_name).map_err(ScrapingError::Http)
+}
+
+pub fn serve(file_name: &str) -> Result<(Vec<u8>, String), ScrapingError> {
+    Repo::read_file(file_name).map_err(ScrapingError::Http)
+}